A theoretical model of the conductivity of a thin metal layer placed in a longitudinal constant magnetic and alternative electric field is constructed, taking into account the diffuse-mirror boundary conditions. An analytical expression is obtained for the integral conductivity as a function of dimensionless parameters: layer thickness, electric field frequency, magnetic field induction and surface specularity coefficients. The dependences of the layer conductivity on the aforenamed parameters are analyzed. The results are compared with known experimental data.