I always carry a cheap, coiled cable, combination bicycle lock. They fit in a medium size topeak saddle bag without taking up much space.

A real thief will have the tools needed to take whatever bicycle he selects. And some thiefs won't object to sticking a *** in you face to take a bike.

BUT... the average bike thief isn't going to be much more than a bored kid. A cheap lock will protect the bike... and keep a kid out of trouble at the same time.