The latest example came last week when Guotai Junan International Holdings Ltd., the Hong Kong unit of a Chinese securities company, said it had been unable to reach its chairman and chief executive Yim Fung since Nov. 18, sending its shares plummeting 12 percent. [...] cases would be highly unusual for other global financial centers, but have become commonplace in Hong Kong, where the city’s stock exchange allows international investors to access an increasing number of mainland Chinese companies. Hong Kong is a former British colony that’s now a specially administered Chinese region with legal and financial systems that are separate from mainland China’s. Waste disposal company Dongjiang Environmental Co.