The concept of mobility, resulting from an analysis of stationary transport where carrier velocity is limited by scattering phenomena, has been widely used till today in microelectronics as a measurable factor of merit and as a parameter of analytical models developed to predict device performance. If scatterings are still playing a major role in decananometer MOSFET and cannot be neglected, ballistic transport in the channel takes a growing importance as the gate length of MOSFETs tends to the nanometer scale. In this context, the mobility concept may appear as highly questionable.