When I started college, I was pretty nervous about walking around with cash. So I never had it. I basically would go get cash if I knew I was going somewhere and needed it, otherwise I relied on my debit card. By my senior year, even fast food places were taking debit, so the only thing I really needed cash for was the occasional soda from a machine. I have found that I am more likely to spend cash and I think a little more about what I spend on a card. Cash just seems more disposable to me, for whatever reason.
I do think my age plays a part in this though. I rarely have ANY cash, and this scares my parents/grandparents to death. DH doesn't carry cash often either. We're in our early 20s and we have an inherent trust in the banking system and in our cell phones, etc. so we don't feel like emergencies require cash like my family does.