I bought a very light combination cable lock from the local home improvement store for a few dollars. It's about a meter long and weighs just over 100g. I call it my touring lock. In my local downtown area I use a much heavier lock.

An aside: Once I left my loaded touring bike among a crowd of homeless people in front of the library in a large Eastern US city, pretty much planning on some loss of gear. It's a twenty-year-old steel bike with thirty-plus-year old gear so the loss would be manageable. And I hadn't shaved or showered in days. The bike wasn't touched. I have shamefully little experience with the homeless, but I wondered if there was some kind of ethic at work there.