  We present a theoretical study of the conductance of atomic junctions
comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold
electrodes. The aim is to elucidate how the presence of noble gas atoms affects
the electronic transport through metallic atomic-size contacts. Our analysis,
based on density functional theory and including van der Waals interactions,
shows that for the lightest elements (He and Ne) no significant current flows
through the noble gas atoms and their effect is to reduce the conductance of
the junctions by screening the interaction between the gold electrodes. This
explains the observations reported in metallic atomic-size contacts with
adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the
conductance due to the additional current path provided by their valence p
states.
