The classical Drude model works with a damping constant which does not depend on frequency. This is a good approximation in most cases. However, there are situations where the damping of the free carriers exhibits a characteristic dependence on frequency, for example in the case of scattering at charged impurities. A rather simple, but successful choice of the frequency dependence of the damping term is the following:

The function for the damping constant is chosen to change smoothly from a constant at low frequencies to another constant level at high frequency. The transition region is defined by the crossover frequency and the width parameter. A typical case of the frequency dependence of the damping constant is shown below:

The parameters of the extended Drude model are set in the following dialog: