 Longitudinal resistance in a Hall bar

Voltage contacts separated along the current direction measure longitudinal resistance, even when the device is called a Hall bar. For an approximately uniform rectangular strip, $R_{xx}\simeq {\rho }_{xx}L/W$. Transverse contacts measure $R_{xy}$ instead. Shubnikov-de Haas effect oscillations in $R_{xx}$ need not be Hall-resistance plateaus; in the integer quantum Hall effect, longitudinal minima accompany quantized transverse plateaus.