"Senate Rule XXVI establishes minimum quorum requirements for four areas of committee activity. [...] Senate committees have discretion in adjusting the minimum quorum requirements mandated in Rule XXVI so long as they adopt rules that are 'not inconsistent with the Rules of the Senate.' For instance, most committees allow a single Senator to take testimony at a hearing rather than the one-third required by Rule XXVI. Committees cannot change the majority physically present quorum needed for reporting a measure. Proxy voting is allowed under Senate rules on a vote to report a matter, but a majority must be physically present and proxy votes cannot make a difference in successfully ordering a measure reported."
CRS Report for Congress, 98-775
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html