  $In$ $situ$ angle-resolved photoemission spectroscopy (ARPES) has been
performed on SrVO$_3$ ultrathin films, which show metallic quantum well (QW)
states, to unveil the origin of the anomalous mass enhancement in the QW
subbands. The line-shape analysis of the ARPES spectra reveals that the
strength of the electron correlation increases as the subband bottom energy
approaches the Fermi level. These results indicate that the anomalous
subband-dependent mass enhancement mainly arises from the quasi-one-dimensional
character of confined V $3d$ states as a result of their orbital-selective
quantization.
