  Due to Pauli blocking of intermediate states, the scattering matrix (or $T$
matrix) of two fermionic atoms in a Fermi gas becomes different from that of
two atoms in free space. This effect becomes particularly important near a
Feshbach resonance, where the interaction in free space is very strong but
becomes effectively suppressed in the medium. We calculate the in-medium $T$
matrix in ladder approximation and study its effects on the properties of
collective modes of a trapped gas in the normal-fluid phase. We introduce the
in-medium interaction on both sides of the Boltzmann equation, namely in the
calculation of the mean field and in the calculation of the collision rate.
This allows us to explain the observed upward shift of the frequency of the
quadrupole mode in the collisionless regime. By including the mean field, we
also improve considerably the agreement with the measured temperature
dependence of frequency and damping rate of the scissors mode, whereas the use
of the in-medium cross section deteriorates the description, in agreement with
previous work.
