String or Duct Tape?

Both. Even my smallest kit has 50' of nylon cord, 6' of duct tape, and 3' of wire. Cordage is one of the hardest things to manufacture in the field but one of the most useful when it comes to making shelter. That said, when your down jacket tears, it's worthless. I wrap duct tape around everything so it's always available. I have 3-4' on each ski pole, I have 10' wrapped on my avalanche shovel, I have 3-4' on my seat post, I have 2-3' wrapped around my pocket first aid kit... it's everywhere, but try building a shelter out of four feet of the stuff and you'll be hard pressed.