  The light emission from a scanning tunneling microscope operated on a Ag(111)
surface at 6 K is analyzed from low conductances to values approaching the
conductance quantum. Optical spectra recorded at a sample voltages V reveal
emission with photon energies hv> 2eV. A model of electrons interacting
coherently via a localized plasmon-polariton mode reproduces the experimental
data, in particular the kinks in the spectra at eV and 2eV as well as the
scaling of the intensity at low and intermediate conductances.
