  We describe a phase-coherent multifrequency lock-in measurement technique
that uses the inverse Fourier transform to reconstruct the nonlinear
current-voltage characteristic (IVC) of a nanoscale junction. The method
provides for a separation of the galvanic and displacement currents in the
junction, and easy cancellation of the parasitic displacement current from the
measurement leads. These two features allow us to overcome traditional
limitations imposed by the low conductance of the junction and high capacitance
of the leads, thus providing an increase in measurement speed of several orders
of magnitude. We demonstrate the method in the context of conductive atomic
force microscopy, acquiring IVCs at every pixel while scanning at standard
imaging speed.
