Nonlinear Response in the Metallic Field Effect

We observe that charging the interface of a silver film on mica at a frequency $\omega$ modulates the conductance of the film at both $\omega$ and $2\omega$. Relative magnitudes and phases of the observed modulations, and their dependence on a superimposed dc charge, are consistent with a field-effect model assuming scattering of conduction electrons by surface charges, if the latter occur in patches that scatter the electrons coherently.