This article has been downloaded from IOPscience. (http://iopscience.iop.org/0295-5075/53/4/544) is available Download details: IP Address: 130.251.188.1 The article was downloaded on 06/11/2007 at 14:13 Please note that terms and conditions apply. More related content HOME | SEARCH | PACS & MSC | JOURNALS | ABOUT | CONTACT US