where in calculating the left current we need I 1 L m together with both I 2 L m and I 2 R m whereas for the right current I 1 L m is replaced by I 1 R m . 

Acknowledgement 1 M.Tahir would like to acknowledge the support of the Pakistan Higher Education Commission (HEC). 

∗ E-Mail: a.mackinnon@imperial.ac.uk 

1 A. Schliesser, et. al., Nature Physics 5, 509 (2009);, K. L. Ekinci and M. L. Roukes, Review of Scientific Instruments 76, 061101 (2005).; K. L. Ekinci, Small 2005,1, No. 8-9, 786-797.; M. L. Roukes, Technical Digest of the 2000 Solid State Sensor and Actuator Workshop; “Nanoelec- tromechanical Systems”.; H. G. Craighead, Science 290, 1532 (2000).; P. Kim and C. M. Lieber, Science 126, 2148 (1999). 

2 S. D. Bennett, and A.A. Clerk, Phys. Rev. B 78, 165328 (2008).; S. Akita, Y. Nakayama, S. Mizooka, Y. Takano, T. Okawa, Y. Miyatake, S. Yamanaka, M. Tsuji, and T. Nosaka, Appl. Phys. Lett. 79, 1691 (2001).; A. M. Fennimore, T. D. Yuzvlnsky, W. Q. Han, M. S. Fuhrer, J. Cummings, and A. Zettl, Nature 424, 408 (2003). 

3 J. Kinaret, T. Nord, and S. Viefers, Appl. Phys. Lett. 82, 1287 (2003).; C.-H. Ke and H. D. Espinosa, Appl. Phys. Lett. 85, 681 (2004).; V. Sazonova, Y. Yaish, H. ¨ Ust¨ unel, D. Roundy, T. Arias, and P. McEuen, Nature 431, 284 (2004). 

4 M. P. Blencowe, Phys. Rep. 395, 159 (2004).; A. N. Cleland, Foundations of Nanomechanics, 2003 (Berlin: Springer). 

5 H. Park et al., Nature (London) 407, 57 (2000).J. Koch and F. von Oppen, Phys. Rev. Lett. 94, 206804 (2005).; J. Koch, M. E. Raikh, and F. von Oppen, ibid. 95, 056801 (2005).; J. Koch, F. von Oppen, and A. V. Andreev, Phys. Rev. B 74, 205438 (2006). 

6 M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, and J. M. Tour, Science 278, 252 (1997).; R. H. M. Smit, Y. Noat, C. Untiedt, N. D. Lang, M. C. van Hemert, and J. M. van Ruitenbeek, ibid. 419, 906 (2002). 

7 L. H. Yu, Z. K. Keane, J. W. Ciszek, L. Cheng, M. P. Stewart, J. M. Tour, and D. Natelson, Phys. Rev. Lett. 93, 266802 (2004).; L. H. Yu and D. Natelson, Nano Lett. 4, 79 (2004).; M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C. von H¨ anisch, F. Weigend, F. Evers, H. B. Weber, 

