Novel, simple techniques were developed to fabricate very narrow GaAs conducting wires by utilizing direct focused ion beam (FIB) implantation. We employed two methods for fabrication of the wires. The first method makes use of high‐resistivity (HR) regions formed by FIB implantation without successive annealing to define a very narrow conducting wire (HR method). In the second one, focused Si ions are line implanted into p‐type GaAs, forming an n‐type conducting wire in the p‐type region. Then, reverse‐bias is applied across the pn junction in order to make the wire narrower by expanding the depletion layer (PN method). This structure has an advantage that the thickness of the wire can be varied by bias voltage. Magnetoconductances measured in all the fabricated wires show evidences of weak electron localization and conductance fluctuations due to a quantum interference effect. The minimum widths of the wires are evaluated to be ∼20 nm (HR method) and ∼100 nm (PN method) by fitting the theory of one‐dimensional weak localization to the experimental data. The phase coherent length of the electronic waves in the wires is also derived to be ∼120 nm at 1.3 K.