  Nucleon exchange mechanism is investigated in central collisions of symmetric
heavy-ions in the basis of the stochastic mean-field approach. Quantal
diffusion coefficients for nucleon exchange are calculated by including
non-Markovian effects and shell structure. Variances of fragment mass
distributions are calculated in central collisions of ${}^{40}$Ca +
${}^{40}$Ca, ${}^{48}$Ca + ${}^{48}$Ca and ${}^{56}$Ni + ${}^{56}$Ni systems.
