We have measured the conductance of Cu nanocontacts for biases V-b = 0.1-0.8 V and investigated the bias dependence of the formation probability p(Cu) of Cu single-atom contacts (SACs). We found that p(Cu) decreases with increasing Vb and vanishes at a critical bias V-bc similar to 0.7 V, contrary to V-bc similar to 2.3 V for p(Au) of Au SACs. Both p(Cu) and p(Au), however, show the same bias dependence when plotted against the reduced bias V-b/V-bc. (C) 2004 Elsevier B.V. All rights reserved.