  The uniform field assumption used to derive semi-classical cutoff energies of
$10U_p$ for electron emission and $3.17U_p$ for high harmonic generation is
applicable for ponderomotive amplitudes ($\propto E\lambda^2$) much smaller
than the field drop-off scale. For large wavelength and high field experiments
at nanoscale structures this assumption may break down by predicting energies
beyond the true classical energy limits. Here we provide generalized
calculations for these cutoff energies by taking into account the spatial field
drop-off. The modified cutoff energies vary significantly from the uniform
field results even with ponderomotive amplitudes still an order of magnitude
below the field drop-off scale. Electron emission and scattering energy as a
function of the time-of-ionization is considered for the nanotip ($\sim1/r^2$)
field profile. The cutoff energies as a function of the adiabaticity parameter
$\delta$, which may be easily calculated for given wavelength, apex field
strength, and nanostructure scale, are then determined through maximization for
nanotip, nanoblade ($\sim1/r$), and exponential field profiles. These profiles
deviate from each other in electron emission energy by up to nearly a factor of
the ponderomotive energy, indicating the importance of mid-field profile
behavior. The electron emission energy cutoff also attains an additional factor
of $U_p$ due to the smooth integrated ponderomotive force in the adiabatic
drop-off and very long pulse regime. These results also provided as
double-exponential fits for ease of use. We then compare the nanoblade electron
emission cutoffs with a quantum simulation of the electron rescattering
process. We also consider a short (few-cycle) pulsed field, focusing on a
cosine-like pulse and overviewing the general carrier-envelope phase
dependencies.
