We report the electron doping in the surface vicinity of KTaO_3 by inducing oxygen-vacancies via Ar^+ -irradiation. The doped electrons have high mobility (> 10^4 cm^2/Vs) at low temperatures, and exhibit Shubnikov-de Haas oscillations with both two-
and three-dimensional components. A disparity of the extracted in-plane effective mass, compared to the bulk values, suggests mixing of the orbital characters. Our observations demonstrate that Ar^+ -irradiation serves as a flexible tool to study low dimensional quantum transport in 5d semiconducting oxides.
