  Resistivity, Hall effect and magnetoresistance have been investigated
systematically on single crystals of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ranging from
undoped to optimally doped regions. A systematic evolution of the quasiparticle
scattering has been observed. It is found that the resistivity in the normal
state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ is insensitive to the potassium doping
concentration, which is very different from the electron doped counterpart
Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, where the resistivity at 300 K reduces to
half value of the undoped one when the system is optimally doped. In stark
contrast, the Hall coefficient R$_H$ changes suddenly from a negative value in
the undoped sample to a positive one with slight K-doping, and it keeps
lowering with further doping. We interpret this dichotomy due to the asymmetric
scattering rate in the hole and the electron pockets with much higher mobility
of the latter. The magnetoresistivity shows also a non-monotonic doping
dependence indicating an anomalous feature at about 80 K to 100 K, even in the
optimally doped sample, which is associated with a possible pseudogap feature.
In the low temperature region, it seems that the resistivity has the similar
values when superconductivity sets in disregarding the different T$_c$ values,
which indicates a novel mechanism of the superconductivity. A linear feature of
resistivity $\rho_{ab}$ vs. $T$ was observed just above $T_c$ for the optimally
doped sample, suggesting a quantum criticality.
