Electrical transport through constrictions in the charge-density wave conductor NbSe3

We have investigated the electrical transport properties of insulating and metallic constrictions of dimensions 100nm-10_m in the charge-density wave (CDW) conductor NbSe3. The constrictions are made in a variety of ways: focused ion beam, reactive ion etching through a resist mask, and in a mechanicallycontrolled break junction configuration. We find that the behaviour of the junctions is independent of the fabrication method, and, depending on the size of the constriction, that the low-temperature behaviour of the constrictions is metallic or insulating. At 4.2K we observe peaks in the differential conductance near 100mV and 200mV, which vanish as the temperature is increased above the Peierls temperatures 59K and 145K respectively. The data is consistent with the interpretation that there are two different regimes, depending on the size of the constriction, demonstrating either tunnelling behaviour, due to CDW-insulator-CDW junctions, or metallic behaviour, due to two normal metal-CDW junctions in series.