  We find that layered materials composed of various oxides of cations with
$s^2$ electronic configuration, $XY_2$O$_4$, $X$=In or Sc, $Y$=Ga, Zn, Al, Cd
and/or Mg, exhibit isotropic electron effective mass which can be obtained via
averaging over those of the corresponding single-cation oxide constituents.
This effect is due to a hybrid nature of the conduction band formed from the
s-states of {\it all} cations and the oxygen p-states. Moreover, the observed
insensitivity of the electron effective mass to the oxygen coordination and to
the distortions in the cation-oxygen chains suggests that similar behavior can
be expected in technologically important amorphous state. These findings
significantly broaden the range of materials as efficient transparent conductor
hosts.
