  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, 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. 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.
