We derive a simple, semi-classical formula for the calculation of the linear electron conductivity spectrum of semiconductor nanocrystals in the THz spectral range from the general quantum-mechanical model. We show that the reliability and range of applicability of the formula significantly exceed the scope of both the Drude-Smith model and the recently developed modified Drude-Smith model. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement