  After making a cold weld by pressing two clean metal surfaces together, upon
gradually separating the two pieces a metallic nanowire is formed, which
progressively thins down to a single atom before contact is lost. In previous
experiments [1,2] we have observed that the stability of such nanowires is
influenced by electronic shell filling effects, in analogy to shell effects in
metal clusters [3]. For sodium and potassium at larger diameters there is a
crossover to crystalline wires with shell-closings corresponding to the
completion of additional atomic layers. This observation completes the analogy
between shell effects observed for clusters and nanowires.
