  In this work, Rayleigh microwave scattering was utilized to measure the
electron number density produced by nanosecond high voltage breakdown in air
between two electrodes in a pin-to-pin configuration (peak voltage 26 kV and
pulse duration 55 ns). The peak electron density decreased from 1*10^17 cm^-3
down to 7*10^14 cm^-3 when increasing the gap distance from 2 to 8 mm (total
electron number decreased from 2*10^13 down to 5*10^11 respectively). Electron
number density decayed on the timescale of about several microseconds due to
dissociative recombination.
