  By angle-resolved photoemission spectroscopy and polarization-dependent
infrared reflectivity measurements the electronic and vibrational properties of
low-dimensional perovskites Sr$_{1-y}$La$_y$NbO$_{3.5-x}$ were studied along
different crystal directions. The electronic behavior strongly depends on the
oxygen and La content, including quasi-one-dimensional metallic and
ferroelectric insulating behavior. An extremely small energy gap at the Fermi
level is found for SrNbO$_{3.41}$ and SrNbO$_{3.45}$ along the conducting
direction at low temperature, suggestive for a Peierls-type instability.
Despite the similar nominal carrier density, for
Sr$_{0.8}$La$_{0.2}$NbO$_{3.50}$ the quasi-one-dimensional metallic character
is suppressed and no gap opening is observed, which can be explained by
differences in the crystal structure. Electron-phonon interaction appears to
play an important role in this series of compounds.
