  The material BaBiO$_{3}$ is known for its insulating character. However, for
thin films, in the ultra-thin limit, metallicity is expected because
BaBiO$_{3}$ is suggested to return to its undistorted cubic phase where the
oxygen octahedra breathing mode will be suppresse as reported recently. Here,
we confirm the influence of the oxygen breathing mode on the size of the band
gap. The electronic properties of a BaBiO$_{3}$ thickness series are studied
using \textit{in-situ} scanning tunneling microscopy. We observe a wide-gap
($E_\textrm{G}$~$>$ 1.2 V) to small-gap~($E_\textrm{G}$~$\approx$ 0.07 eV)
semiconductor transition as a function of a decreasing BaBiO$_{3}$ film
thickness. However, even for an ultra-thin BaBiO$_{3}$ film, no metallic state
is present. The dependence of the band gap size is found to be coinciding with
the intensity of the Raman response of the breathing phonon mode as a function
of thickness.
