Motivated by recent experiments on electrochemically controlled Pb atomic-scale switches we have studied the self-diffusion of Pb on flat and stepped surfaces since diffusion processes play an important role in the growth of metal substrates. Kinetic modelling based on Monte-Carlo simulations using a model potential suggests that exchange processes play an important role in the contact formation at the nanoscale. Periodic density functional theory indeed find that the barriers for exchange diffusion across the steps are significantly lower than for hopping diffusion. The consequences for the contact formation in electrochemically controlled switches are discussed. (C) 2014 Elsevier Ltd. All rights reserved.