The quantum states of a system of particles in a finite spatial domain in general consist of a set of discrete energy eigenvalues; these are usually grouped into bunches of degenerate or close-lying levels(1), called shells. In fermionic systems, this gives rise to a local minimum in the total energy when all the states of a given shell are occupied, In particular, the closed-shell electronic configuration of the noble gases produces their exceptional stability. Shell effects have previously been observed for protons and neutrons in nuclei, and for clusters of metal atoms(2-4). Here we report the observation of shell effects in an open system-a sodium metal nanowire connecting two bulk sodium metal electrodes, which are progressively pulled apart. We measure oscillations in the statistical distribution of conductance values, for contact cross-sections containing up to a hundred atoms or more. The period follows the law expected from shell-closure effects, similar to the abundance peaks at 'magic' numbers of atoms in metal clusters(3,4).
