  Perovskite alkaline earth stannates, such as $BaSnO_3$ and $SrSnO_3$, showing
light transparency and high electrical conductivity (when doped), have become
promising candidates for novel optoelectrical devices. Such devices are mostly
based on hetero-structures and understanding of their electronic structure,
which usually deviates from the bulk, is mandatory for exploring a full
application potential. Employing angle-resolved photoemission spectroscopy and
ab initio calculations we reveal the existence of a 2-dimensional metallic
state at the $SnO_2$-terminated surface of a 1\% La-doped $BaSnO_3$ thin film.
The observed surface state is characterized by distinct carrier density and a
smaller effective mass in comparison with the corresponding bulk values. The
small surface effective mass of about $0.12m_e$ can cause an improvement of the
electrical conductivity of BSO based heterostructures.
