We use more than 900 million linked employer–employee records covering the entire formal workforce of Brazil to examine when cities provide upward mobility for the initially poor. We find that upward mobility for low-wage workers is strongest in workplaces that combine low-wage individuals with high-wage workers. This association is particularly pronounced in Brazil’s southern cities, where more complex industries include workers with a wider range of skills. The association is weaker in northern cities, where formal employment is dominated by the public sector.

Here is the paper by Radu Barza, Edward Glaeser, César A. Hidalgo, and Martina Viarengo.   And here is a useful thread on the paper.  This is one of the best and most important economics papers I have seen in the last few years.