The authors report on measurements of the phase coherence time, tau _phi, as determined from the measurement of weak negative magnetoresistance in narrow pinched Si accumulation layers. Under favourable bias conditions, one-dimensional quantum interference and electron interaction corrections to the conductivity are found. The phase coherence length is then best described in terms of the 1D Nyquist phase-breaking mechanism, with a Landau-Baber (pure metal limit) component which retains its 2D form.