The anomalous degradation of the low-field mobility observed in short-channel metal-oxide-semiconductor field-effect transistors is analyzed by collating various reported data in experiments and simulations. It is inferred that the degradation is not caused by the channel scattering of the carriers. The origin is proposed to be the backscattering of channel carriers on injection into the drain. The expression of the low-field mobility, including the backscattering effect, is derived. The inverse of the low-field mobility is a linear function of the inverse of channel length, the expression of which reproduces that empirically derived by Bidal's group. By fitting the expression to simulated as well as experimental data, we can estimate the value of parameters related to the channel scattering and also to the backscattering from the drain. We find that these values are in reasonable magnitude.