  An explicit perturbative computation of the Mori's memory function was
performed by G\"otze and W\"olfle (GW) to calculate Generalized Drude
scattering (GDS) rate for the case of electron-impurity and electron-phonon
scattering in metals by assuming constant electronic density of states at the
Fermi energy. In the present investigation, we go beyond this assumption and
extend the GW formalism to the case in which there is a gap around the Fermi
surface in electron density of states. The resulting GDS is compared with a
recent one by Sharapov and Carbotte (SC) obtained through a different route. We
find good agreement between the two at finite frequencies. However, we find
discrepancies in the dc scattering rate. These are due to a crucial assumption
made in SC namely $\omega >> \vert \Sigma(\epsilon+\omega) -
\Sigma^{*}(\epsilon)\vert$. No such high frequency assumption is made in the
memory function based technique.
