We hired Michael Yang.  He is an English speaking tour guide.  I emailed him from the US and he promptly replied to my email.  He asked about choices for drivers, vehicle and what we wanted to do.  He was able to accommodate all of our needs and he was flexible during the day.  We even got to bike around the City wall in Xian. 

His knowledge of the Terra Cotta Warriors and the city of Xian made it a very informative day.