AbstractAn energy‐loss‐method formalism applicable to a thin quantized film is developed. Within the frame of a self‐consistent field and taking into account the “imaginary” charges in the film surrounding regions the expression for a film dielectric function is derived and, in the long‐wavelength limit the analytical form of its imaginary part is found for a simple model of a size quantized film. Under the condition that the electron gas is nondegenerate and only the first subband is filled, the mobility is calculated, when an electron‐ionized impurity scattering is taken into account, and compared with the relevant Boltzma