Don't argue that you weren't doing 50. It is a looser. Especially if he was using laser.
Call Caltrans and find out if they have the Engineering Traffic Speed survey for that section of road. If they don't, ask them who does. Get it and determine if the speed limit is justified (they rarely are). If not, then it was a speed trap.
Look up VC40800 - 40805 and read up on speed traps while you are wating for your survey to come in. Also... make sure you post here what the survey says. There are a few other things for you to consider.

