These work well, but they're a little hard to get started:
Pocket chain saw
And they last forever. One of the handle doohickies on mine broke so I replaced both handles with webbing straps.
You could pick up one of those little wire jobbies at your local sporting goods store for a couple bucks. They don't last long and are a pita, but they're samll and cheap.
If I had the room, I'd just carry a hacksaw.