Electron mobility in bulk Si n-type MOSFETs with the gate length between 30 nm and 740 nm was determined at room temperature by magnetoresistance measurements. The decrease of the mobility with the gate length was analysed within a model taking into account the ballistic motion and an increased scattering in the pockets. We show that these effects are of a comparable magnitude for strong inversion. The transmission coefficient is determined showing a large fraction of ballistic electrons in short channel devices. (c) 2006 Elsevier Ltd. All rights reserved.