Hello, I have an unlocked iPhone 5 in Canada. I'm traveling to the US for a few weeks and I'd like to pick up a SIM and activate prepaid. I've done this in the past with AT&T micro SIMs. Is there such a thing as an AT&T nano SIM that will let me activate on AT&T prepaid (GoPhone I think it's called?) If not, is there an alternative carrier with whom I could do this?