  The optical conductivity $\sigma(\omega)$ and the d.c. resistivity $\rho(T)$
within the extended t-J model on a square lattice, as relevant to high-$T_c$
cuprates, are reinvestigated using the exact-diagonalization method for small
systems, improved by performing a twisted boundary condition averaging. The
influence of the next-nearest-neighbor hopping $t'$ is also considered. The
behaviour of results at intermediate doping is consistent with a
marginal-Fermi-liquid scenario and in the case of $t'=0$ for $\omega>T$ follows
the power law $\sigma \propto \omega^{-\nu}$ with $\nu \sim 0.65$ consistent
with experiments. At low doping $c_h<0.1$ for $T<J$ $\sigma(\omega)$ develops a
shoulder at $\omega\sim \omega^*$, consistent with the observed mid-infrared
peak in experiments, accompanied by a shallow dip for $\omega < \omega^*$. This
region is characterized by the resistivity saturation, whereas a more coherent
transport appears at $T < T^*$ producing a more pronounced decrease in
$\rho(T)$. The behavior of the normalized resistivity $c_h \rho(T)$ is within a
factor of 2 quantitatively consistent with experiments in cuprates.
