  By inserting a SrZrO$_3$ buffer layer between the film and the substrate, we
demonstrate a significant reduction of the threading dislocation density with
an associated improvement of the electron mobility in La:BaSnO$_3$ films. A
room temperature mobility of 140 cm$^2$ V$^{-1}\text{s}^{-1}$ is achieved for
25-nm-thick films without any post-growth treatment. The density of threading
dislocations is only $4.9\times 10^{9}$ cm$^{-2}$ for buffered films prepared
on (110) TbScO$_3$ substrates by pulsed laser deposition.
