  We report on systematic and high-precision measurements of dephasing, an
effect that fundamentally limits the performance of laser wakefield
accelerators. Utilizing shock-front injection, a technique providing stable,
tunable and high-quality electron bunches, acceleration and deceleration of
few-MeV quasi-monoenergetic beams were measured with sub-5-fs and 8-fs laser
pulses. Typical density dependent electron energy evolution with 65-300
micrometers dephasing length and 6-20 MeV peak energy was observed and is well
described with a simple model.
