  We study high-order harmonic generation (HHG) in model atoms driven by
plasmonic-enhanced fields. These fields result from the illumination of
plasmonic nanostructures by few-cycle laser pulses. We demonstrate that the
spatial inhomogeneous character of the laser electric field, in a form of
Gaussian-shaped functions, leads to an unexpected relationship between the HHG
cutoff and the laser wavelength. Precise description of the spatial form of the
plasmonic-enhanced field allows us to predict this relationship. We combine the
numerical solutions of the time-dependent Schr\"odinger equation (TDSE) with
the plasmonic-enhanced electric fields obtained from 3D finite element
simulations. We additionally employ classical simulations to supplement the
TDSE outcomes and characterize the extended HHG spectra by means of their
associated electron trajectories. A proper definition of the spatially
inhomogeneous laser electric field is instrumental to accurately describe the
underlying physics of HHG driven by plasmonic-enhanced fields. This
characterization opens new perspectives for HHG control with various
experimental nano-setups
