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.
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