  In-plane resistivity, magnetoresistance and Hall effect measurements have
been conducted on quenched K$_x$Fe$_{2-y}$Se$_2$ single crystals in order to
analysis the normal-state transport properties. It is found that the Kohler's
rule is well obeyed below about 80 K, but clearly violated above 80 K.
Measurements of the Hall coefficient reveal a strong but non-monotonic
temperature dependence with a maximum at about 80 K, in contrast to any other
FeAs-based superconductors. With the two-band model analysis on the Hall
coefficient, we conclude that a gap may open below 65 K. The data above 65 K
are interpreted as a temperature induced crossover from a metallic state at a
low temperature to an orbital-selective Mott phase at a high temperature. This
is consistent with the recent data of angle resolved photoemission
spectroscopy. These results call for a refined theoretical understanding,
especially when the hole pockets are absent or become trivial in
K$_x$Fe$_{2-y}$Se$_2$ superconductors.
