  Employing Nernst-Einstein decomposition $\sigma=e^2\chi_c D$ of the
conductivity $\sigma$ onto charge susceptibility (compressibility) $\chi_c$ and
diffusion constant $D$, we argue that the bad-metallic behavior of $\sigma$ in
the regime of high temperatures and lightly doped insulator is dominated by the
strong temperature and doping dependence of $\chi_c$. In particular, we show
how at small dopings $\chi_c$ strongly decreases towards undoped-insulating
values with increasing temperature and discuss simple picture leading to the
linear-in-temperature resistivity with the prefactor increasing inversely with
decreasing concentration ($p$) of doped holes, $\rho\propto T/p$. On the other
hand, $D$ shows weak temperature and doping dependence in the corresponding
regime. We support our arguments by numerical results on the prototypical two
dimensional Hubbard model and discuss the proposed picture at length from the
experimental point of view.
