  Electron charge transport through a quantum point contact (QPC) driven by an
asymmetric spin bias is studied. A large charge current is induced when the
transmission coefficient of the QPC jumps from one integer plateau to the next.
Furthermore, for an open external circuit, the induced charge bias instead of
the charge current is found to be quite large. It provides an efficient and
practical way to detect spin bias by using a very simple device, a QPC or a STM
tip. In addition, with the aid of magnetic field, polarization direction of the
spin bias can also be determined.
