Two pure mean field theories, the classical Vlasov- and the quantal TDHF-approach, are used to study 85 MeV/N 12C induced reactions with targets from 12C to 197Au. Both theories predict nearly identical results: the nuclei slip through each other; for C+C about 85% of the longitudinal momentum is conserved in the surviving fragments. Inclusion of two body collisions via an Uehling-Uhlenbeck collision integral yields - in sharp contrast to results at lower energies - two clearly distinct components: only 20% of the initial momentum resides in the slipped-through fragments, which retain only 40% of the nucleons. The remaining 60% of the nucleons undergo one or more n-n scatterings and form a mid-rapidity source. These nucleons decelerate rapidly with a time constant t = 12.5 fm/c, corresponding to a deceleration length of x = 2.5 fm. The number of slipped-through nucleons decreases even further when heavier target nuclei are studied: for C+Au collisions, 97% of the projectile nucleons undergo at least one collision, transferring 80% of the total longitudinal momentum to the target-like fragment. The number of emitted uncollided projectile nucleons falls off exponentially with the thickness of the target nucleus, yielding a mean free path of the nucleon of λ = 2.6 fm.
