  We investigated the transport properties of BaFe$_2$As$_2$ single crystals
before and after annealing with BaAs powder. The annealing remarkably improves
transport properties, in particular the magnitude of residual resistivity which
decreases by a factor of more than 10. From the resistivity measurement on
detwinned crystals we found that the anisotropy of the in-plane resistivity is
remarkably diminished after annealing, indicative of dominant contributions to
the charge transport from the carriers with isotropic and high mobility below
magnetostructural transition temperature $T_{\rm s}$ and the absence of nematic
state above $T_{\rm s}$. We found that the Hall resistivity shows strong
non-linearity against magnetic field and the magnetoresistance becomes very
large at low temperatures. These results give evidence for the manifestation of
multiple carriers with distinct characters in the ordered phase below $T_{\rm
s}$. By analyzing the magnetic field dependences, we found that at least three
carriers equally contribute to the charge transport in the ordered phase, which
is in good agreement with the results of recent quantum oscillation
measurements.
