CA is a mighty big place, and I haven't traveled all that much of it. However I do happen to have phones on ATT's 3G network that can act as hotspots, and USB networking devices for VZW and Sprint. I don't have T-Moble because the coverage map looked like it wasn't really useful.
I have no data for Nevada. Last time I was in Arizona I didn't have a VZW device, but ATT seemed fine pretty much everywhere.
If you want AT&T's network but don't like the price, H2OWireless uses ATTs network and has better prices than ATT prepaid...but don't bother calling tech support unless you have 3 hours to burn (hiring a single person for tech support calls must be how they keep costs low!).