Dephasing of conduction electrons due to zero-point fluctuation

We consider conduction-electron dephasing by the zero-point fluctuation of an electromagnetic field in a vacuum. We argue that there is an upper cutof f frequency of the zero-point fluctuation mode beyond which the zero-point fluctuation will not contribute to the dephasing of conduction electrons. T he upper cutoff frequency is material-parameter dependent. We find that the dephasing time is proportional to the electron diffusion constant. This re sult agrees very well with existing experimental data.