  The conductivity $\sigma$ and carrier density $n$ of the conducting polymer
polyaniline were investigated by changing the concentration $x$ of a secondary
dopant, meta-cresol. We found that $\sigma$ changes by four orders of magnitude
within the $x$-range of 1-10 %, while $n$, as estimated from the Hall
measurements, shows a weak dependence on $x$ in the region of 2% <$x$< 50%.
These results suggest that $\sigma$ can be enhanced by the change in the
mobility $\mu$. We analyzed the temperature dependence of $\mu$ by not only the
combination of two different types of scattering mechanism, but also by the
polaron hopping model. The experimental data of $\mu(T)$ can be explained well
by the latter model with reasonable fitting parameter values of a small-polaron
binding energy and a longitudinal optical phonon frequency.
