Up to now quantized conductance of the carbon nanotube has only been observed by replacing the tip of a scanning probe microscope with the tube and putting the other end of the tube in a liquid metal [Science 280(1998) 1744]. Probably cleaning the tube by the liquid metal has improved its quality and gives rise to the quantized conductance. We report on a new method to improve the electric transport properties of a single multi-wall carbon nanotube by Joule heating. Our experiment indicates that the conductance of the tube can be greatly improved just by repeated scanning the bias voltage in vacuum.
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