The following is from Hewett (ed.), "Texas Confederate Soldiers," Vol. I, Name Roster (Wilmington, NC: Broadfoot Publ., 1997). There is no William Leftwich listed. The only Capt. Nelson is Capt. Governeur H. Nelson, 1st (McCulloch's) Cav. Company E. I would suspect that the Leftwich you are looking for was a member of the Texas State Troops (state militia), which were not actually in Confederate service. A check in the Texas State Archives and rosters which may exist in Red River County, Texas, would probably be your best bet.