Using a new extraction methodology taking into account multisubband population and carrier degeneracy, we have experimentally determined backscattering coefficients, ballistic ratios, and injection velocities of n- and p-FDSOI devices with gate lengths down to 30 nm in the saturated and, for the first time, in the linear regimes. The evolution of these quasi-ballistic parameters is examined as a function of the inversion charge in the channel and at temperatures ranging from 50 to 293 K, showing stronger ballistic ratios in the saturated regime than in the linear one. We particularly focus on the linear regime, and a model linking ballisticity ratios and effective mobility is proposed and validated experimentally for different gate lengths. According to the experimental evaluation of the device mean-free path and its evolution with both the inversion charge in the channel and the temperature, we investigate the mobility degradation with decreasing gate lengths, highlighting the importance of Coulomb scattering on this unexpected mobility behavior.