  It is shown on the basis of the Fermi liquid theory that the s-wave
scattering potential due to nonmagnetic impurity is strongly enhanced by the
mass enhancement factor $1/z$. As a result the impurity potential with moderate
strength, of the order of the bandwidth of conduction electrons, gives
scattering in the unitarity limit in strongly correlated metals as heavy
fermions. This effect is embodied by large pressure dependence of residual
resistivities in heavy fermion systems, since the pressure decreases the degree
of correlations, which makes the Kondo temperature increase rapidly. This
accounts for the large pressure dependence of the residual resistivity observed
in heavy fermion metals, such as CeInCu$_{2}$, CeCu$_{6}$, CeAl$_{3}$.
