We have measured 50 magnetoconductance curves corresponding to 50 microscopic disorder configurations in a single mesoscopic GaAs:Si wire at T=45 mK. This permits to establish that the universal conductance fluctuation decreases by a factor-of-two when one breaks time reversal symmetry by application of the magnetic field which destroys the weak-localisation correction to the mean conductance. We have also observed the same factor of reduction for the 1/f noise power in a gated GaAs:Si wire at T=45 mK, that indicates the quantum interference origin of this noise at least at very low temperature