  Topology is a key ingredient driving the emergence of quantum devices.
Topological field-effect transistor (TFET) has been proposed to outperform the
conventional FET by replacing the ON state with topology-protected quantized
conductance, while the OFF state remains the same normal insulating
characteristics and hence bears similar drawbacks. Here, we demonstrate a
proof-of-concept TFET having both ON and OFF quantized conductance, by
switching between helical and chiral topological screw dislocation (SD) states
in three-dimensional topological insulators. A pair of SDs are configured with
one acting as channel and the other as gate controlled by local magnetic field.
A reversible field-switching is achieved with the ON and OFF conductance of
$2e^2/h$ and $e^2/h$, respectively, as shown by tight-binding quantum transport
calculations. Furthermore, BaBiO$_3$ is shown as a candidate material having
the desired topological SD states, based on first-principles calculations. Our
findings open a new route to high-fidelity topological quantum devices.
