Three-terminal devices with conduction channels formed by quasi-metallic carbon nanotubes (CNTs) are shown to operate as nanotube-basedfield-effect transistors under strong magnetic fields. The off-state conductance of the devices varies exponentially with the magnetic fluxintensity. We extract the quasi-metallic CNT chirality as well as the characteristics of the Schottky barriers formed at the metal-nanotubecontacts from the temperature-dependent magnetoconductance measurements.