An attempt is made to investigate the effects of size quantization on the Einstein relation for the diffusivity‐mobility ratio in ultrathin films of n‐Cd3As2. It is found that the ratio increases, at a given electron concentration, with decreasing film thickness and also increases with increasing electron concentration corresponding to a given film thickness. For extremely thin films, the influence of size quantization becomes increasingly prominent with decreasing values of the electron concentration. Copyright © 1981 WILEY‐VCH Verlag GmbH & Co. KGaA