  We present results demonstrating the space charge-mediated transition between
classical, diffusion-limited current and surface-conduction dominant
over-limiting current in a shallow micro-nanochannel device. The extended space
charge layer develops at the depleted micro-nanochannel entrance at high
current and is correlated with a distinctive maximum in the dc resistance.
Experimental results for a shallow surface-conduction dominated system are
compared with theoretical models, allowing estimates of the effective surface
charge at high voltage to be obtained. In comparison to an equilibrium estimate
of the surface charge obtained from electrochemical impedance spectroscopy, it
is further observed that the effective surface charge appears to change under
applied voltage.
