  We probe the electron transport properties in the shell of GaAs/In0.2Ga0.8As
core/shell nanowires at high electric fields using optical pump / THz probe
spectroscopy with broadband THz pulses and peak electric fields up to 0.6
MV/cm. The plasmon resonance of the photoexcited charge carriers exhibits a
systematic redshift and a suppression of its spectral weight for THz driving
fields exceeding 0.4 MV/cm. This behavior is attributed to the intervalley
electron scattering resulting in the increase of the average electron effective
mass and the corresponding decrease of the electron mobility by about 2 times
at the highest fields. We demonstrate that the increase of the effective mass
is non-uniform along the nanowires and takes place mainly in their middle part,
leading to a spatially inhomogeneous carrier response. Our results quantify the
nonlinear transport regime in GaAs-based nanowires and show their high
potential for development of nano-devices operating at THz frequencies.
