  We theoretically examine the widespread hypothesis of an electro-viscous
origin of the increase in apparent viscosity observed in recent experiments on
capillary filling of nanochannels. Including Debye-layer corrections to the
hydraulic resistance we find that the apparent viscosity reaches a maximum in
the mesoscopic regime where the channel height (or more generally the hydraulic
radius) is comparable to the screening length. However, for realistic estimates
of central parameters, we find that the electro-viscous contribution to the
apparent viscosity is at most a 1% effect.
