Kryptonite makes those small cables. They are quite strong.

There is almost nothing that will deter a dedicated thief, so, at least, make them work for it

One of the magazine type programs on TV had a story about bike thiefs a couple years ago. They planted a bike, with a homing beeper in the handlebars, and waited for someone to steal the bike. With a hidden camera they watched a guy, carrying a messenger bag, go up to the bike, take out of the bag a pair of bolt cutters, cut the chain and ride off in about 15 seconds. They followed him and nailed him after he sold it to a receiver of stolen goods. As I recall they didn't have him arrested, but interviewed him for their program. (He said he wouldn't do it any more, now that he had been caught.)