Young adults are generally hard to survey, presenting researchers with numerous difficulties. They are hard to locate and contact due to high mobility. They are hard to persuade and exhibit high levels of resistance to survey participation. As a result, they pose a greater challenge for longitudinal surveys. This paper explores the role of mode of data collection in young adults' decisions to stay in a longitudinal panel. We draw on data from the National Young Adult Health Survey (NYAHS). NYAHS is a longitudinal study (three annual waves and 2 brief between-wave follow-up surveys) of adults aged 18-34 initially recruited in 2019 through RDD sampling of cell phone numbers nationwide. All sampled cell phone numbers were randomly assigned to one of three experimental conditions; the conditions differed in mode of data collection used in subsequent interviews once screened in. In the first condition, young adults continue all rounds of interviews by telephone ("telephone only" condition). The second group of young adults completed one round of interview by web and the rest by telephone ("telephone mostly" condition). The last third was asked to complete three interviews online and two interviews by telephone ("web mostly" condition). We examined the impact of mode switching on young adults' likelihood of participating in later surveys and on nonresponse bias in key survey outcomes. We found that switching young adults from telephone to web had an immediate negative effect on their likelihood of participating in that web survey, but it did not have a continued negative effect. Switching them from web to telephone increased response rates and reduced nonresponse bias. The findings have important practical implications on how to survey young adults.