Formation of a Metallic Contact: Jump to Contact Revisited
  The transition from tunneling to metallic contact between two surfaces does not always involve a jump, but can be smooth. We have observed that the configuration and material composition of the electrodes before contact largely determines the presence or absence of a jump. Moreover, when jumps are found preferential values of conductance have been identified. Through combination of experiments, molecular dynamics, and first-principles transport calculations these conductance values are identified with atomic contacts of either monomers, dimers or double-bond contacts. 
C. Untiedt, M.J. Caturla, M.R. Calvo, J.J. Palacios, R.C. Segers, and   J.M. van Ruitenbeek