  We use optimal fluctuation method for a new ballistic $\sigma$-model to study
the long time dispersion of conductance $G(t)$ of a mesoscopic sample. In the
long time limit the conductance of a $d$-dimensional sample decays as $\exp (-A
\ln^d t ) $. At shorter times the new results match those in our previous
paper. It is found that at very long times the diffraction effects are
important and the ballistic treatment is not valid. We also suggest a physical
picture of trapping.
