  Using dynamical-mean-field theory, we investigate the electronic properties
of quantum wells consisting of a $t^1_{2g}$-electron system with strong
correlations. The special focus is on the subband structure of such quantum
wells. The effective mass is found to increase with increase in the value of
the bottom of the subband, i.e., decrease in the subband occupation number.
This is due to the combination of Coulomb repulsion, whose effect is enhanced
on surface layers, and longer-range hoppings. We discuss the implication of
these results for the recent angle-resolved photoemission experiment on
SrVO$_3$ thin films.
