A numerical calculation method is proposed to compute the transport mean free path of single-mode semiconductor nanosheets with surface roughness, where the mean free path is extracted from the statistical average of the logarithm of the dimensionless resistance. The present method requires only a computationally less demanding coherent transmission probability, and is applicable to wider channel length ranges from the quasi-ballistic to the localization regime. The channel thickness, T w, dependence of the mean free path calculated by the proposed method within the effective mass approximation shows the well-known T W 6 dependence for thicker T w, while it becomes weaker for thinner T w.