As an engineering undergrad (EE) wanting to switch to physics for a PhD, I also spent some time trying to answer this question. It's hard to find a single answer, as it depends so much on what you want to study, the school, and your actual background (research? projects? etc...). Your situation is different then mine, as I only spent 2 years in industry, and you've been in software for 13. But I can tell you what I found. I don't think that 13 years is a problem, might mean that some math is a bit rusty, but you've certainly learned other skills that would be very useful in graduate school.
Your best bet is to contact some schools directly, (I used the contact info from the AIP data on the gradschoolshopper website). Most had some language about wanting an undergraduate degree in physics "or equivalent", or some wording like that. For the top schools, they've got enough excellent applications from students with very thorough physics backgrounds, so it is best to be realistic about where you want to go. There are many very good programs that aren't necessarily in the national spot light for being "top ranked".
If you don't have a physics degree, I figure that the schools will at least want to know that you're capable of doing basic physics, a good score on the physics GRE will help a lot here. Having a full time job meant that I had plenty of time to study for the PGRE, and I took advantage of that.
So in short, no, you don't need a physics bachelors. Seems like your math and electronics should help. If you're a good student, I think schools will be more interested in your abilities, then making sure that you have all the credits for a bachelor's in physics. My school offers the possibility to make up deficiencies in undergraduate physics background. I considered taking the undergraduate mechanics course, because I had never been exposed to Lagrangians or Hamiltonians as an EE, but I found that with some work, I got through the graduate mechanics course fine, and really enjoyed it.
Material science and physics seems to overlap a fair bit, I'm considering working with a physics advisor who collaborates closely with someone in the material science department. They work on the theory of phase transitions of various things. My physics department seems to be fairly interdisciplinary, although they used to be more so. Their current stance is that PhD research should be primarily physics, and if it isn't, then the student is expected to transfer to the other department. So it would probably help to have a good, justified reason for wanting to switch areas. Why physics, specifically, instead of more material science?
There are some other threads on this site about switching from engineering to physics, some might be related to your situation.