  We present an experimental and theoretical study of the conductance and
stability of Mg atomic-sized contacts. Using Mechanically Controllable Break
Junctions (MCBJ), we have observed that the room temperature conductance
histograms exhibit a series of peaks, which suggests the existence of a shell
effect. Its periodicity, however, cannot be simply explained in terms of either
an atomic or electronic shell effect. We have also found that at room
temperature, contacts of the diameter of a single atom are absent. A possible
interpretation could be the occurrence of a metal-to-insulator transition as
the contact radius is reduced, in analogy with what it is known in the context
of Mg clusters. However, our first principle calculations show that while an
infinite linear chain can be insulating, Mg wires with larger atomic
coordinations, as in realistic atomic contacts, are alwaysmetallic. Finally, at
liquid helium temperature our measurements show that the conductance histogram
is dominated by a pronounced peak at the quantum of conductance. This is in
good agreement with our calculations based on a tight-binding model that
indicate that the conductance of a Mg one-atom contact is dominated by a single
fully open conduction channel.
