Due to a new quasi-ballistic extraction methodology dedicated to low-longitudinal-field conditions, experimental carrier mean-free-paths have been determined on strained and un-strained fully depleted silicon-on-insulator (n-FDSOI) devices with Si film thickness ranging from 11.8 to 2.5 run, gate length down to 30 nm, and a TiN/HfO2 gate stack. Electron mobility evolution with the Si film thickness, reported in a previous study, is explored and quantitatively explained. Moreover, through inversion charge and temperature deep investigations, dominant carrier transport mechanisms are analyzed. It is experimentally revealed that transport degradation occurs in short and thin channels, which is shown to be mainly due to additional Coulomb scatterings rather than ballistic artifact in both strained and unstrained devices.