The effect of laser-focusing conditions on the evolution of relativistic plasma waves in laser-wakefield accelerators is studied both experimentally and with particle-in-cell simulations. For short focal-length (w(0)lambda(p)), a single optical filament can capture the majority of the laser energy and self-guide over distances comparable to the dephasing length, even for these short pulses (c tau approximate to lambda(p)). This allows the wakefield to evolve to the correct shape for the production of the monoenergetic electron bunches, as measured in the experiment.