Soldiers, for example, are to leave after demobilization, many construction workers are just there for road or railway projects, some officials are assigned to work in Tibet on a rotation basis and then leave.
While some business people operate stores or restaurants there, they seldom intend a long-term stay, he said.
"Once you leave Lhasa, you hardly meet any Han Chinese,"said Nentwig, who spent a month in Tibet for a field research on yak shepherds in the summer of 2002.
"I did my field research in a county where just 20 or 30 Han Chinese live among 50,000 to 60,000 Tibetans," he said.
The scholar said the overall proportion of long-term Han residents in Tibet is about just seven percent, while ethnic Tibetans account for over 90 percent.
Even taking the short-term residents into account, the Han people account for an estimated 20 to 25 percent of the entire population in Tibet, while ethnic Tibetans are still the "overwhelming majority of about 75 to 80 percent,"he said.
Areas inhabited by ethnic Tibetans in the neighboring provinces of Qinghai, Gansu, Sichuan and Yunnan, however, are ethically and culturally more diversified, where Tibetans have coexisted peacefully with Han and other ethnic groups such as Hui, Mongolian, Qiang, Tu and Salar for many centuries, Nentwig said.