  Recently, quantum corrections to optical conductivity of disordered metals up
to the UV region were observed. Although this increase of conductivity with
frequency, also called anti-Drude behaviour, should disappear at the electron
collision frequency, such transition has never been observed or described
theoretically. Thus, the knowledge of optical conductivity in a wide frequency
range is of great interest. It is well known that the extrapolation of complex
conductivity is ill-posed - a solution of the analytic continuation problem is
not unique for data with finite accuracy. However, we show that assuming
physically appropriate properties of the searched function $\sigma(\omega)$,
such as: symmetry, smoothness, and asymptotic solution for low and high
frequencies, one can significantly restrict the set of solutions. We present a
simple numerical method utilizing the radial basis function approximation and
simulated annealing, which reasonably extrapolates the optical conductivity
from visible frequency range down to far infrared and up to ultraviolet region.
Extrapolation obtained on MoC and NbN thin films was checked by transmission
measurement across a wide frequency range.
