  We investigate the gate-voltage dependence of the magnetoconductivity of
several amorphous InGaZnO$_4$ (a-IGZO) thin-film transistors (TFTs). The
magnetoconductivity exhibits gate-voltage- controlled competitions between weak
localization (WL) and weak antilocalization (WAL), and the respective weights
of WL and WAL contributions demonstrate an intriguing universal dependence on
the channel conductivity regardless of the difference in the electrical
characteristics of the a-IGZO TFTs. Our findings help build a theoretical
interpretation of the competing WL and WAL observed in the electron systems in
a-IGZO TFTs.
