I carry a ski rack lock - absolutely minimal combo lock with a retractable 3' cable, the size of a pack of cigarettes. It's just enough to stop someone from walking off with the bike on a whim. Where I ride, at least where I would be out of sight of the bike for a couple of minutes, that's about all I need. Otherwise the bike is inside, or lashed with pounds of chain in the back of the truck. Locking depends so much on your circumstance.

KeS