1. Condensed Matter Physics 114-36, California Institute of Technology, Pasadena, California 91125, USA
2. Present address: Department of Physics, University of Utah, Salt Lake City, Utah 84112 , USA

Correspondence to: M. L. Roukes¹ Correspondence and requests for materials should be addressed to M.L.R. (e-mail: Email: roukes@caltech.edu).

Abstract

The physics of mesoscopic electronic systems has been explored for more than 15 years. Mesoscopic phenomena in transport processes occur when the wavelength or the coherence length of the carriers becomes comparable to, or larger than, the sample dimensions. One striking result in this domain is the quantization of electrical conduction, observed in a quasi-one-dimensional constriction formed between reservoirs of two-dimensional electron gas^{1, 2}. The conductance of this system is determined by the number of participating quantum states or 'channels' within the constriction; in the ideal case, each spin-degenerate channel contributes a quantized unit of 2e²/h to the electrical conductance. It has been speculated that similar behaviour should be observable for thermal transport^{3, 4} in mesoscopic phonon systems. But experiments attempted in this regime have so far yielded inconclusive results^{5, 6, 7, 8, 9}. Here we report the observation of a quantized limiting value for the thermal conductance, G_th, in suspended insulating nanostructures at very low temperatures. The behaviour we observe is consistent with predictions^{10, 11} for phonon transport in a ballistic, one-dimensional channel: at low temperatures, G_th approaches a maximum value of g₀ = ²k ²_BT/3h, the universal quantum of thermal conductance.

1. Condensed Matter Physics 114-36, California Institute of Technology, Pasadena, California 91125, USA
2. Present address: Department of Physics, University of Utah, Salt Lake City, Utah 84112 , USA

Correspondence to: M. L. Roukes¹ Correspondence and requests for materials should be addressed to M.L.R. (e-mail: Email: roukes@caltech.edu).