A photoconductive response time, as short as 350 fs, of a low-temperature-grown GaAs (LT-GaAs) micro-coplanar photoconductive switch has been measured and modelled to the ultrafast trapping of the photoexcited carriers in LT-GaAs. The coherent interference of the pump and probe pulses results in a narrow spike photocurrent autocorrelation signal which maps the femtosecond optical pulses.